Source with kind permission of Paul Nixon Number Prefixes more complete information and explanation appears there.
Many army numbers carried letter prefixes, a lot of these being introduced during the First World War. Some prefixes, like the letter “G”, used to prefix the numbers of men in various Home Counties battalions who enlisted during 1914-1918 for General Service, were very common; others less so. Prefixes were normally expressed on army paperwork as G/1234 or G1234 and may be represented on medals as G-1234, however the use of prefixes was often inconsistent.
Number prefixes were also used by some regiments to identify the particular battalion that a man served with. The 3/ and 4/ prefixes are common for Special Reserve and Extra Reserve recruits, and during World War One, many service battalions (the so-called "Pals' Battalions" in particular) followed suit. For example, a man serving with the 15th (Service) Battalion (1st Leeds) West Yorkshire Regiment, would have his number expressed as 15/1234 – usually.
The following list of letter prefixes is not exhaustive but it could provide clues as to which unit or battalion a man served with.
Army Service Numbers - Letter Prefixes
A - Army Service Corps (old Special Reserve).
A - King’s Royal Rifle Corps (WW1). Numbers A/1 to A/3919 were issued to time-expired soldiers who were enlisting for one year's service. This prefix is however, also found on men's records who had no prior army service. Later in the war numbers A/200000 to A/205598 were allocated to men who had been posted from the Training Reserve
A - Royal Scots Fusiliers, 3rd (Reserve) Battalion.
A - Army Ordnance Corps
A - Cameronians (Scottish Rifles)
A - King's Own Scottish Borderers, 3rd (Reserve) Battalion
A - Highland Light Infantry 3rd (Reserve) Battalion
A (HT) - Army Service Corps, Horse Transport Special Reserve
ARMR - Army Ordnance Corps Armourer
ASC - Army Service Corps
B - The Rifle Brigade (WW1). Numbers B/1 to B/3514 were issued to time-expired soldiers who were enlisting for one year's service.
B - Army Service Corps, Special Reserve
B - Royal Fusiliers, 26th (Bankers) Battalion
B - Scottish Rifles, 4th (Extra Reserve) Battalion
B (HT) - Army Service Corps, Horse Transport Special Reserve Territorial Force (TF)
C - King's Royal Rifle Corps (WW1). Some 1914-1918 service battalions, starting from the 16th (Church Lads) Battalion but extending to the 21st (Yeoman Rifles) inclusive
C - Royal Munster Fusiliers, 1st Garrison Battalion (Cork)
C (MT) - Army Service Corps, Mechanical Transport Special Reserve
CAT - Army Service Corps, Caterpillar Mechanical Transport
CH - Royal Marines, Chatham Division Royal Marine Light Infantry
CHT - Army Service Corps, Corps of Horse Transport
CMT - Army Service Corps, Corps of Motor Transport
D - Dragoons & Dragoon Guards
D – Royal Dublin Fusiliers
DG - Dragoon Guards
DEAL - Royal Marines, Deal Depot
DEPOT / D - Royal Marine Light Infantry. Deal Depot Permanent Staff
DM - Army Service Corps, Driver, Mechanical Transport.
DM2 - Army Service Corps, Mechanical Transport Learners. Discontinued in November 1916.
E - Royal Fusiliers, 17th (Empire) Battalion
E - Army Service Corps, Forage Department
EKent - East Kent Regiment (prior to the 1902 introduction of the L/ prefix for this regiment)
ES - East Surrey Regiment (prior to the 1902 introduction of the L/ prefix for this regiment)
F – Middlesex Regiment, 17th and 23rd (Football) Battalions
F - Army Service Corps, Forage Department
G - Royal Irish Fusiliers. 1st, 2nd and 3rd Garrison Battalions.
G - Found on various Home Counties’ Regiments’ Service battalions, WW1
G - Royal Fusiliers; General Service enlistment, WW1
G4 - Found on some WW1 enlistments into the 4th (Extra Reserve) Bn, East Surrey Regt
GS – General Service enlistment, WW1
GS - Cavalry regiments; General Service enlistment, WW1
GSR - General Special Reserve; WW1
GS/RS - General Service Royal Sussex (see Albert Henry Gaston as an example)
GSSR - General Service Special Reserve, WW1. Certainly used by The Royal Sussex Regiment and the Royal West Kent Regiment
GYM – Army Gymnastic Staff
H - Hussars
H - North Irish Horse
J - Royal Fusiliers 38th, 39th 40th, 42nd (Judean) Battalions
K - Royal Lancaster Regt, The King's Own (WW1)
K - King’s (Liverpool) Regiment, 1914 enlistments, 11th - 14th Battalions
K - Royal Fusiliers 22nd (Kensington) Battalion
L - Lancers
L - Regular enlistments. I have always assumed that L represents "Line" but I would be pleased to stand corrected. This prefix first starts appearing in 1902 and is found on regular enlistments into the following regiments: The Buffs (East Kent Regiment), East Surrey Regiment, Middlesex Regiment, Queen's (Royal West Surrey Regiment), Royal Fusiliers, Royal Sussex Regiment and the Royal West Kent Regiment. All of these regiments, with the exception of the Royal Fusiliers, formed the Number 10 Grouped Regimental District. My thanks to David Langley for his inputs on this prefix.
L - Royal Artillery,; First World War volunteer enlistment, the 'L' standing for 'Local' or locally raised; the L/ prefix commonly seen on New Army divisional artillery regimental numbers
L - (King's) Liverpool Regiment
LSR - Royal Sussex Regiment Special Reserve, enlisted under regular terms
M - Army Service Corps. Mechanical Transport
M1 & M2 - Army Service Corps
M2 - Army Service Corps. Electricians
MS – Army Service Corps. Mechanical Specials
MT – Army Service Corps. Mechanical Transport
N - National Reserve (East Surrey Regiment amongst others)
O - Rifle Brigade. Numbers O/1 to O/984 did not appear at the front until 1917.
O – Army Ordnance Corps
P – Rifle Brigade, 16th (Service) Battalion (St Pancras) and the 17th (Reserve) Battalion which was a reserve battalion for the 16th.
P - Military Foot Police & Military Mounted Police
P - Dragoon Guards
PET - Army Service Corps. Petroleum Dept
PLY - Royal Marines Plymouth Division, Royal Marine Light Infantry
PLY/RMP - Royal Marine Police. Plymouth
PO - Royal Marines. Portsmouth Division, Royal Marine Light Infantry
PO/RMP - Royal Marine Police. Portsmouth
PS - Royal Fusiliers. 18th, 19th, 20th & 21st (Public Schools) Battalions
PS - Middlesex Regiment. 16th (Public Schools) Battalion
PW - Middlesex Regiment. 18th, 19th & 26th (Public Works) Battalions (Later Labour Corps)
R - Army Service Corps. Remounts
R - King's Royal Rifle Corps. New Army men within the range R/1 to R/58003
RMA - Royal Marines. Royal Marine Artillery
RMB - Royal Marines. Royal Marine Band
RME - Royal Marines. Royal Marine Engineers
RS and R/TS - Army Service Corps. Remount Specials
RS - Royal Sussex (see Albert Henry Gaston as an example)
RX - Army Service Corps Army Remount Section
S - Army Service Corps. Supply Branch
S - Devonshire Regiment (allegedly, although I have personally not come across any S/ prefix Devons numbers)
S - Royal Army Medical Corps
S - Royal Artillery
S - Highland Regiments, WW1 enlistment
S - Rifle Brigade, WW1 enlistment
S - Royal Munster Fusiliers
S - Army Ordnance Corps
S1, 2, 3, 4 - Army Service Corps. 1st/2nd/3rd/4th New Armies Supply (S4 Labour)
SB –Royal Fusiliers. 10th (Stockbrokers) Battalion
SD –Royal Sussex Regiment. 11th, 12th and 13th (South Down) Battalions and the 14th (Reserve) Battalion.
SE - Army Veterinary Corps (Special Enlistment for duration of 1914-1918 War)
SP - Royal Fusiliers. 23rd & 24th (Sportsman's) Battalions
SPTS – Royal Fusiliers. 23rd & 24th (Sportsman's) Battalions
SR – Special Reserve
SRGS - Special Reserve General Service (found on some East Surrey Regiment records)
SR/GSES - Special Reserve / General Service East Surrey (WW1 enlistments into the East Surrey Special and Extra Reserve)
SR3 - Special Reserve (found on some East Surrey Regiment records)
SS – Army Service Corps. Supply Special (butchers, bakers, clerks etc)
SRMT – Army Service Corps. Special Reserve Motor Transport
STK – Royal Fusiliers. 10th (Stockbrokers) Battalion
T - Army Service Corps. Horse Transport
T - Army Ordnance Corps
T - Territorial Force
T1 & 2/ (SR) - Army Service Corps. Enlisted Special Reserve for New Armies
T1, T2, T3 & T4 - Army Service Corps. Horse Transport
TF - Territorial Force.
TR – Training Reserve
TS – Army Service Corps. Transport Specials (trade)
TSR – Army Service Corps . Transport Special Reserve
TZ - Royal Naval Volunteer Reserve. Tyne Division 1914-1918 enlistment
W - Royal Naval Volunteer Reserve, Wales Division
W – Cheshire Regiment, 13th (Service) Battalion (Wirral)
W - 38th (Welsh) Division, Royal Artillery
W - Military Prison Staff Corps 1902-1906 (Army Order 241. December 1901)
W - Military Provost Staff Corps (Army Order 114. 1906).
WF - Royal Welsh Fusiliers. Appears on some attestation papers from the mid 1890s
WR - Royal Engineers. (Waterways & Railways)
Y - King’s Royal Rifle Corps (WW1). Y/1 to Y/1905 were issued to men enlisting for one year as Special Reservists and men enlisting for three years’ short service.
Z - Rifle Brigade (WW1). Numbers Z/1 to Z/2997 were issued to men enlisting for one year as Special Reservists and men enlisting for three years’ short service.